1. Field of the Invention
The invention relates to a method of producing a metallic shaped part, in particular a vehicle body part, from a semifinished product made of an unhardened hot-workable steel sheet.
2. Related Art of the Invention
Many parts, in particular body parts in vehicle construction, must satisfy stringent requirements with regard to rigidity and strength. At the same time, in the interests of weight reduction, the parts are to have as small a material thickness as possible. In order to meet these two requirements, high-strength and super-high-strength steel materials, which—depending on composition and heat treatment—have very high strength, are being increasingly used. The production of vehicle body parts from these super-high-strength steel sheets is preferably effected in a hot-forming process, in which—as described, for example, in DE 100 49 660 A1—a sheet blank is heated and then shaped in a special shaping tool and hardened. In this case, by the process parameters during the hot forming being suitably selected, the strength and toughness values of the part can be specifically set.
To produce such a part by means of hot forming, first of all a sheet blank is cut out of a coil, this sheet blank is then heated above the structural transformation temperature of the steel materials, above which the material structure is in the austenitic state, is inserted in the heated state into a forming tool and formed into the desired part shape and is cooled down while mechanically fixing the desired forming state, tempering or hardening of the part being effected.
However, in order to cut a part produced in this way in a dimensionally accurate manner, a large outlay in terms of equipment is required: in particular, very high cutting forces are required for the cold cutting of hardened materials, which leads to rapid tool wear and high maintenance costs. Furthermore, the cold trimming of such high-strength parts is problematical, since, for example, the part edges trimmed in the cold state have more or less large burrs, a factor which may lead to rapid crack formation in the part on account of the high notch sensitivity of the high-strength materials.
To avoid these difficulties which occur during the mechanical trimming of the hardened parts, alternative cutting methods are often used, such as, for example, laser cutting or water-jet cutting. High-quality trimming of the edge of the parts can certainly be achieved by means of these methods, but these cutting methods work comparatively slowly, since the cycle times here depend directly on the length of the cut edge and on the tolerances to be maintained. The final trimming process therefore produces a bottleneck during the production of hot-formed parts, which limits the number of parts to be produced per unit of time. The total cycle time of the part production can certainly be reduced if—depending on the length of the cut edge—a plurality of laser or water-jet cutting units working in parallel are provided; however, this involves high additional investment and logistics outlay and is therefore disadvantageous.